962 FXUS65 KPIH 100314 AFDPIH Area Forecast Discussion National Weather Service Pocatello ID 914 PM MDT Tue Oct 9 2018 .UPDATE...Local web cameras show some rain and snow mixed falling in the Arco Desert area this evening. Forecasts were updated to reflect slightly lower snow levels, but with little accumulation. RS && .PREV DISCUSSION... /issued 230 PM MDT Tue Oct 9 2018/ SHORT TERM...Tonight through Thursday. A low pressure system is centered over western Idaho early this afternoon. This low will move through southern Idaho and into northern Utah this evening. A large batch of precipitation is moving through the region this afternoon and will continue through the night. The heaviest precipitation will fall across southern Idaho with models indicating that there may be a break in the precipitation across portions of the central mountains into the upper Snake highlands later tonight. Snow levels have been problematic so far, generally running about 500 feet lower than what the 850mb wet bulb zero has been indicating. As a result, made an adjustment to snow levels through the remainder of the event to hopefully account for it. Most of the snow and associated accumulations should fall above 6000 feet, although snow is possible as low as 5000 feet in some locations extending from the central mountains through the upper Snake Plain and into the upper Snake highlands. From an impacts standpoint, we could see some slick roads across extreme southeast Idaho tonight and into Wednesday morning where up to 3 inches is forecast even on valley floors. Mountain passes across southeast Idaho will likely see snow with 2 to 4 inches likely. Locations like Emigration Summit could potentially see noticeably higher amounts, possibly as much as 7 or 8 inches. For these reasons, will issue a winter weather advisory for these areas. Meanwhile, an additional 1 to 3 inches is likely above 6000 feet tonight around Monida Pass which could create impacts there as well. A winter weather advisory will be issued for that area as well. Elsewhere, we anticipate additional snows to be minor with little or no impacts. By Wednesday afternoon, much of the precipitation across the southeast corner will be breaking up while additional showers are expected to develop across the central Idaho mountains through the upper Snake Plain into the upper Snake highlands. This is in response to an upper level trough that is moving into the region, which is helping to drive the low away from here. Potentially, locations above 6000 feet across the central mountains and upper Snake highlands could see a couple more inches of snow during Wednesday afternoon and night. Finally, Thursday the precipitation should begin to diminish across the region as high pressure gradually builds in. Valle LONG TERM...Friday Through Tuesday. Generally quieter weather is expected in the long-term period. Dry conditions under NW flow on the west side of long-wave troughing over the Central US is expected Fri with some sunshine. A shortwave riding down the western side of the larger trough may bring some shower activity to the Central Mntns, Upper Snake Highlands, Teton Valley, Caribou Highlands, and Bear Lake region Sat, with some support for this scenario present across the latest GFS/EC/Canadian runs. Overall precip amounts do not look terribly heavy this far out, but most of this activity will likely fall as snow with cold temps in place. Dry and clear/sunny weather is then expected Sun- Tues as high pressure attempts to nudge into the Pacific NW. Forecast confidence is moderate throughout the period with decent overall model agreement. The biggest story may actually be those cold temperatures...highs may barely reach 50 in the Snake Plain and valleys of the Southern Highlands, and may only reach the upper 40s Sun. If these temps verify, a widespread hard freeze with low temps in the low-mid 20s at lower elevations will definitively end the growing season by Sun morning. - KSmith/Valle AVIATION...A low pressure storm system will continue to produce widespread rain and rain/snow shower activity across SE Idaho this afternoon into tonight. MVFR cigs are expected at KPIH/KIDA/KBYI likely lingering into Wed, although have indicated a slight improving trend Wed in the TAFs as rain shower activity gradually tapers off late Wed AM/Wed afternoon and allows cigs to start to improve. Vsbys should be less of an issue and generally VFR, especially considering temps will largely support rain, although some brief reductions to 4-5 SM cannot be ruled out during periods of steadier rain/mist, especially at KIDA. At KDIJ, guidance is generally running (perhaps suspiciously) optimistic. Rain should increase this afternoon and eve, before tapering off a bit overnight. Will likely see at least high-end MVFR cigs with this activity, so sent decks to 3,000 feet despite guidance running a bit higher. Most of the precip seems to wind down before temps fall too low tonight, but any lingering shower activity overnight and Wed will need to be watched for vsby reductions due to snow. Finally, KSUN will continue to experience the most significant impacts from this storm. Cigs have run 500-900 feet for the last 7 hours, which are at or below airport minimums. After a brief break in precip, the next round has moved in and will continue into early tonight with continued IFR cigs and vsbys occasionally flirting with IFR criteria as well. Really see no indications conditions will significantly improve through the rest of the day, so cigs below airfield minimums are forecast to continue through around 04Z before a gradual improving trend ensues overnight/Thurs AM as precip chances back off and winds may shift into the NW as per their usual diurnal cycle. Looking ahead, more progressive improvement is expected region-wide by Thurs eve as the storm system pulls away, which should send all terminals to VFR with clearing skies. We may need to watch for fog overnight into Fri morning. - KSmith/Valle FIRE WEATHER...Low pressure overhead has layed down some significant rain and snow, and will continue for all areas through tonight. A very slow progression eastward allows the western zones to start drying out Thu afternoon, and for most other locations on Fri. A weak trough brings back a threat of precipitation to the ID- MT border region, including some windy conditions in the Snake River plain and ridges of central and eastern Idaho. The upper level ridging strengthens on Sun, pushing the storm track farther north and keeping it there at least into mid-week of next week. Messick && .PIH WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until 10 AM MDT Wednesday for IDZ060>063- 067. && $$